The use of chlorine dioxide is well established as an antimicrobial compound for various industrial applications. It is widely used for sanitation of food as well as food processing equipment. The common foods that are treated with chlorine dioxide are red meat, poultry, seafood, potatoes, mushrooms, fruits and vegetables. A number of water treatment plants globally use chlorine dioxide for the disinfection of drinking water.
In comparison to other commonly used antimicrobials such as glutaraldehyde, sodium hypochorite, chloramines and quaternary ammonium compounds, chlorine dioxide has a much higher rate of microbial kill. Furthermore, unlike chlorine (Cl2) or hypochlorites (OCl−), chlorine dioxide does not form carcinogenic byproducts such as trihalomethanes. The ultimate breakdown byproduct of chlorine dioxide is innocuous sodium chloride (NaCl). Therefore, ClO2 is generally a primary chemical of choice for applications of environmental concern.
The U.S. Environmental Protection Agency is increasingly expressing its concern towards the use of harmful chemicals used in oilfield production. The agency has made it mandatory to publicly disclose the chemicals used in production of oil and natural gas. Therefore, the use of chlorine dioxide is growing because of its low toxicity profile.
A process which is an the integral part of the oil and gas production is known as hydraulic fracturing. In this process, the well is rigorously stimulated for the purpose of hydrocarbon recovery from the subsurface. It is generally performed in nano-darcy subterranean formations where average permeability is below 1×10−6 darcy (darcy is the unit of permeability).
The process involves pumping of a fluid at high pressure and high velocity through a vertical and often a horizontal section of a well. The well is installed with a casing with perforations at targeted intervals of the subterranean formation that holds the hydrocarbon. The pressure exerted during the fracturing process is greater than the pressure required to fracture the formation. During the pumping and after confirmation that fracturing has been initiated, a proppant (such as sand or ceramic beads) is added into the frac water to hold open those fractures to maintain the pathways and prevent subsequent collapse.
Water is the most common fluid that is used for the fracturing process. Water is mixed with friction reducers of desired viscosity to optimize fracturing. This hydraulic mix is prone to microbial growth, which must be inhibited in order to keep the formation free of biological contamination. Many types of bacteria exist in surface water and in the pits, pipes, pumps and tanks used to store and transport that water for the fracturing. If this microbial contamination is not controlled, it can lead to a range of expensive problems for energy companies. These problems include reduced production due to plugging with biofilm or biogenic sulfidic scales, deterioration of the quality of the oil or gas due to the presence of bacteria generated hydrogen sulfide or carbon dioxide, as well as corrosion caused by acid producing or sulfate reducing bacteria.
Use of antimicrobials, also known as biocides, has now become a standard practice in the frac or well treatment water in an effort to minimize contamination by surface bacteria.
Chlorine dioxide is commonly used as a biocide to control the bacteria in the frac water because it is known to be effective against all the types of bacteria of concern to the industry. It breaks down biofilm and polymers where bacteria can hide, and it has a favorable environmental profile. Chlorine dioxide is also used in well treatments because it is effective at neutralizing hydrogen sulfide, breaking down sulfidic scales, and can remove biofilm and polymer residues that can plug the well.
Chlorine dioxide is prohibited from transportation and must be generated on site. At concentrations above 10% in air, chlorine dioxide poses hazards of instantaneous decomposition leading to possible explosion. Therefore it must be produced on site and care must be taken to generate safe concentrations of this gas. It is for this reason that the gas is generated in an aqueous form.
One common method of production of chlorine dioxide is by mixing sodium chlorite and an acid in a predetermined ratio. Often, a third component, sodium hypochlorite, is added as a reactant to improve the yield of chlorine dioxide. The resultant solution has a gaseous content of 50,000-250,0000 ppm which is transferred from generator or storage to the point of injection. Leaks or accidental breakage of the conduits presents risks to an applicator. Therefore these generated solutions are extensively diluted prior to transfer. However, this process only delivers a solution with fixed concentration.
Typical chlorine dioxide generation systems utilize high mixing ratios of activator to sodium chlorite solution to obtain the highest possible conversion to chlorine dioxide. Conversion rates are normally from 80% to 100% depending on whether chlorine is incorporated into the sodium chlorite/acid solution. The chlorine dioxide solution is then injected into the frac or well treatment water.
To this end, although methods of production and delivery of chlorine dioxide of the existing art are operable, further improvements are desirable to enhance the use of a device and method of production of solutions of chlorine dioxide that are safe, effective and convenient to use. It is to such a device, system and method that the present disclosure is directed.